Cement compositions containing degradable materials and methods of cementing in subterranean formations

ABSTRACT

The present invention provides cement compositions that include degradable materials, and methods of using such compositions in subterranean cementing operations. An example of a method of the present invention includes: providing a cement composition that includes a hydraulic cement, and a degradable material; placing the cement composition in a subterranean formation; allowing the cement composition to set therein; and allowing the degradable material to degrade. Another example of a method of the present invention is a method of enhancing the mechanical properties of a cement composition including adding a degradable material to the cement composition and allowing the degradable material to degrade.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.10/802,340, filed Mar. 17, 2004, entitled “Cement CompositionsContaining Degradable Materials And Methods Of Cementing In SubterraneanFormations,” by B. Raghava Reddy, et al., which is incorporated byreference herein for all purposes, from which priority is claimedpursuant to 35 U.S.C. § 120.

BACKGROUND

The present invention relates to methods and compositions for use insubterranean cementing operations. More particularly, the presentinvention relates to cement compositions comprising degradablematerials, and methods of using such compositions in subterraneancementing operations.

Hydraulic cement compositions are commonly utilized in subterraneanoperations, particularly subterranean well completion and remedialoperations. For example, hydraulic cement compositions are used inprimary cementing operations whereby pipe strings such as casings andliners are cemented in well bores. In performing primary cementing,hydraulic cement compositions are pumped into an annular space betweenthe walls of a well bore and the exterior surface of a pipe stringdisposed therein. To ensure that the annular space is completely filled,a cement slurry is pumped into the annular space until the slurrycirculates to the surface. The cement composition is then permitted toset in the annular space, thereby forming an annular sheath of hardened,substantially impermeable cement. The hardened cement substantiallysupports and positions the pipe string in the well bore and bonds theexterior surfaces of the pipe string to the walls of the well bore.Hydraulic cement compositions are also used in remedial cementingoperations, such as plugging highly permeable zones or fractures in wellbores, plugging cracks and holes in pipe strings, and the like.

Subterranean formations traversed by well bores naturally may be weak,extensively fractured, and highly permeable. In some cases, if thefracture gradient of the formation is exceeded by the hydrostatic headpressure normally associated with cement pumped into the well bore, theformation will fracture. This may result in the loss of cement into theextensive fractures of the formation. This can be problematic because,inter alia, less cement composition will remain in the annular space toform the protective sheath that bonds the pipe string to the walls ofthe well bore. Accordingly, loss of circulation of the cement slurryinto the formation is of great concern.

Conventional attempts to solve the problem of lost circulation of cementslurries include adding polymeric flakes or film strips that may bridgethe cracks and fractures in the formation and, thus, prevent the loss ofthe cement slurry. Examples of such materials include cellophane flakes,polypropylene flakes, or mica flakes, among others. However, the use ofpolypropylene flakes may be undesirable because the polymer is notbiodegradable. Mineral flakes such as mica often have unsuitable sizesthat preclude their use.

Conventional attempts to solve the problem of inadvertently fracturingthe subterranean formation during cementing operations have alsoinvolved, inter alia, the use of cementing slurries with reduceddensities. For example, cement slurry densities can be desirably reducedby incorporating an expanding additive, such as nitrogen, into thecement composition. Alternatively, lightweight particulate additives,such as hollow glass or ceramic beads, may be incorporated into thecement composition at the surface. However, these methods may beproblematic because, inter alia, they can require elaborate andexpensive equipment, which may not be accessible for use in remoteareas.

Certain conventional cement compositions also may become brittle and/orinelastic at some point after setting into a cement sheath. This may beproblematic because, inter alia, an excessively brittle or inelasticcement sheath may become unable to provide desired zonal isolation, andmay require costly remediative operations. This may be particularlyproblematic in the case of multilateral wells. If the cement sheath inthe area of the junction between a principal well bore and a lateralwell bore in a multilateral well is excessively brittle or inelastic, itmay be unable to withstand impacts that may occur, e.g., when tools usedin drilling and completing the well collide with casing in the junctionarea as the tools are moved in and out of the well.

Well bores that comprise an expandable tubular present another scenariowhere an excessively brittle or inelastic cement sheath may beproblematic. The expansion of an expandable tubular inadvertently maycrush at least a portion of the cement sheath behind the tubular,thereby impairing the cement sheath's ability to provide the desiredzonal isolation.

SUMMARY OF THE INVENTION

The present invention relates to methods and compositions for use insubterranean cementing operations. More particularly, the presentinvention relates to cement compositions comprising degradablematerials, and methods of using such compositions in subterraneancementing operations.

An example of a method of the present invention is a method of cementingin a subterranean formation comprising: providing a cement compositioncomprising a hydraulic cement and a degradable material; placing thecement composition into a subterranean formation; allowing the cementcomposition to set therein; and allowing the degradable material todegrade.

Another example of a method of the present invention is a method ofenhancing the mechanical properties of a cement composition comprisingadding a degradable material to the cement composition and allowing thedegradable material to degrade.

An example of a composition of the present invention is a cementcomposition comprising a hydraulic cement and a degradable material.

Other and further features and advantages of the present invention willbe readily apparent to those skilled in the art upon a reading of thedescription of preferred embodiments which follows.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention relates to methods and compositions for use insubterranean cementing operations. More particularly, the presentinvention relates to cement compositions comprising degradablematerials, and methods of using such compositions in subterraneancementing operations. While the compositions and methods of the presentinvention are useful in a variety of subterranean applications, they areparticularly useful in well completion and remedial operations,including primary cementing, e.g., cementing casings and liners in wellbores, including those in multilateral subterranean wells.

The improved cement compositions of the present invention generallycomprise: a hydraulic cement; at least one degradable material; andwater sufficient to make the cement composition a slurry. Otheradditives suitable for use in conjunction with subterranean cementingoperations also may be added to these compositions if desired. When thecement compositions of the present invention set, the resultant cementsheath may have improved mechanical properties that enhance the cementsheath's ability to sustain cyclic stresses due to temperature andpressure. The cement composition also may have improved thixotropicproperties that may enhance its ability to handle loss of circulationand gas migration during the time in which it sets. In an exemplaryembodiment of the present invention, degradation of the degradablematerial may be accompanied by formation of a new product, e.g., saltsor gases, that may act as expanding additives that will enhance theshrinkage compensation properties and/or elasticity, of the resultantset cement.

Any cement may be utilized in the cement compositions of the presentinvention, including, but not limited to, hydraulic cements comprisingcalcium, aluminum, silicon, oxygen, and/or sulfur, which set and hardenby reaction with water. Examples of suitable hydraulic cements arePortland cements, pozzolanic cements, gypsum cements, high aluminacontent cements, phosphate cements, silica cements, and high alkalinitycements. In certain exemplary embodiments of the present invention, APIPortland Cement Classes A, G, and H are used.

The water used in the present invention may comprise fresh water, saltwater (e.g., water containing one or more salts dissolved therein),brine (e.g., saturated salt water), or seawater. Generally, the watercan be from any source provided that it does not contain an excess ofcompounds that may adversely affect other components in the cementcomposition. The water may be present in an amount sufficient to form apumpable slurry. In certain exemplary embodiments, the water may bepresent in the cement compositions in an amount in the range of fromabout 25% to about 150% by weight of cement (“bwoc”). In certainexemplary embodiments, the water may be present in the cementcompositions in the range of from about 30% to about 75% bwoc.

The cement compositions of the present invention comprise a degradablematerial. For example, the degradable material may be a polymericmaterial capable of degrading into sorbable components while in contactwith the cement compositions of the present invention. The degradablematerial may be present in the cement compositions of the presentinvention in an amount sufficient to result, upon partial or completedegradation of the degradable material, in a resultant set cement havinga desired density and desired mechanical properties (e.g., a desiredYoung's modulus and tensile strength). In certain exemplary embodimentsof the present invention, the degradable material degrades after thecement composition has set in a subterranean formation. In certain otherexemplary embodiments, the degradable material may degrade before orwhile the cement composition sets. In certain exemplary embodiments, thedegradable material may be present in the cement compositions of thepresent invention in an amount in the range of from about 1% to about25% bwoc. In certain exemplary embodiments, the degradable material maybe present in the cement compositions of the present invention in anamount in the range of from about 5% to about 15% bwoc. In choosing theappropriate degradable material, one should consider the degradationproducts that will result. These degradation products should notadversely affect other operations, or properties of the set cementsheath. The choice of degradable material also can depend, at least inpart, on the conditions of the well, e.g., well bore temperature.

Nonlimiting examples of degradable materials that may be used inconjunction with the present invention include, but are not limited to,degradable polymers. Such degradable polymers may be capable ofundergoing an irreversible degradation downhole. In a further exemplaryembodiment, the products of the degradation may be sorbable into thecement matrix. As referred to herein, the term “irreversible” will beunderstood to mean that the degradable material, once degraded downhole,should not reconstitute while downhole, e.g., the degradable materialshould degrade in situ but should not reconstitute in situ. The terms“degradation” or “degradable” refer to both the two relatively extremecases of hydrolytic degradation that the degradable material mayundergo, e.g., bulk erosion and surface erosion, and any stage ofdegradation in between these two. This degradation can be a result of,inter alia, a chemical reaction. The rate at which the chemical reactiontakes place may depend on, inter alia, the chemicals added, temperatureand time. The degradability of a polymer depends at least in part on itsstructure. For instance, the presence of hydrolyzable and/or oxidizablelinkages in the backbone often yields a material that will degrade asdescribed herein. The rates at which such polymers degrade are dependenton factors such as, but not limited to, the type of repetitive unit,composition, sequence, length, molecular geometry, molecular weight,morphology (e.g., crystallinity, size of spherulites, and orientation),hydrophilicity, hydrophobicity, surface area, and additives. The mannerin which the polymer degrades also may be affected by the environment towhich the polymer is exposed, e.g., temperature, presence of moisture,oxygen, microorganisms, enzymes, pH, and the like.

Suitable examples of degradable polymers that may be used in accordancewith the present invention include, but are not limited to, thosedescribed in the publication of Advances in Polymer Science, Vol. 157entitled “Degradable Aliphatic Polyesters,” edited by A.-C. Albertsson,the relevant disclosure of which is incorporated herein by reference.Examples of polyesters that may be used in accordance with the presentinvention include homopolymers, random, block, graft, and star- andhyper-branched aliphatic polyesters.

Another class of suitable degradable polymers that may be used inaccordance with the present invention include polyamides and polyimides.Such polymers may comprise hydrolyzable groups in the polymer backbonethat may hydrolyze under the basic conditions that exist in cementslurries and in a set cement matrix. Such polymers also may generatebyproducts that may become sorbed into the cement matrix. Calcium saltsare a nonlimiting example of such byproducts. Nonlimiting examples ofsuitable polyamides include proteins, polyaminoacids, nylon, andpoly(caprolactam). Another class of polymers that may be suitable foruse in the present invention are those polymers that may containhydrolyzable groups, not in the polymer backbone, but as pendant groups.Hydrolysis of the pendant groups may generate a water-soluble polymerand other byproducts that may become sorbed into the cement composition.A nonlimiting example of such a polymer includes polyvinylacetate, whichupon hydrolysis forms water-soluble polyvinylalcohol and acetate salts.

A variety of processes may be used to prepare the degradable polymersthat are suitable for use in the cement compositions of the presentinvention. Examples of such processes include, but are not limited to,polycondensation reactions, ring-opening polymerizations, free radicalpolymerizations, anionic polymerizations, carbocationic polymerizations,coordinative ring-opening polymerizations, and any other appropriateprocess. Exemplary polymers that may be used in accordance with thepresent invention include, but are not limited to, aliphatic polyesters;poly(lactides); poly(glycolides); poly(ε-caprolactones);poly(hydroxybutyrates); poly(anhydrides); aliphatic poly(carbonates);ortho esters; poly(orthoesters); and poly(vinylacetates). In anexemplary embodiment of the present invention, the degradable materialis poly(vinylacetate) in bead form, commercially available from AldrichChemical Company. In another exemplary embodiment of the presentinvention, the degradable material is poly(lactic acid), commerciallyavailable from Cargill Dow Polymers, LLC.

In certain exemplary embodiments, the rate of degradation of the polymeris such that the unhydrolyzed polymer additive retains its structure andshape until it may be suitable for an intended application. For example,it may be desirable for the polymers of the present invention to remainsubstantially insoluble (e.g., phase-separated) in the slurry until atleast such time as the slurry is placed in a subterranean application.Furthermore, the rate of degradation of the degradable material may bevaried depending on factors such as the hydraulic cement, the degradablematerial chosen, and the subterranean conditions of the application.

Generally, the degradable materials may be present in the cementcomposition in any shape, and may be of any size. In certain exemplaryembodiments, the degradable materials may be spherical, substantiallyspherical, bead-shaped or fiber-shaped. In a further exemplaryembodiment of the present invention, voids in the shape of theindividual particles of the degradable material may form within thecement sheath.

In other embodiments, the rate of degradation of the degradable materialmay be such that a barrier may be formed by the degradable material toprevent slurry loss into a permeable zone (e.g., a zone comprisingfractures). In a further exemplary embodiment, the barrier may remainwithout substantially degrading until the cement has set. In yet afurther exemplary embodiment, the degradable material used to form thebarrier may be flakes or film strips. Examples of such film-forminghydrolyzable polymers include, but are not limited to, polylactic acid,polyvinylacetate, and cellulose acetate. Such polymers also may have theadditional advantage of being biodegradable.

In one exemplary embodiment of the present invention, the degradablematerial may enhance the properties of the cement composition bydegrading to form reactive gases, (e.g., carbon dioxide, sulfur oxide,and the like), and/or by degrading to form salts. In a furtherembodiment, the degradable material may degrade to form gases that reactwith the cement composition to form an insoluble salt. In still afurther embodiment, the gases produced may be inert, and may occupy thespace formerly occupied by the degradable material.

Optionally, the cement compositions of the present invention maycomprise a gas that is added at the surface (e.g., nitrogen) or agas-generating additive that may generate a gas in situ at a desiredtime (e.g., aluminum powder or azodicarbonamide). When included in acement composition of the present invention, aluminum powder maygenerate hydrogen gas in situ, and azodicarbonamide may generatenitrogen gas in situ. Other gases and/or gas-generating additives alsomay be suitable for inclusion in the cement compositions of the presentinvention. The inclusion of the gas or gas-generating additive in thecement compositions of the present invention may allow a cementcomposition to have “tunable” mechanical properties. For example, acement composition of the present invention may be formulated to have adesired initial elasticity or flexibility through inclusion of a gas orgas-generating additive, which elasticity or flexibility then may changeover time to a second desired value through degradation of thedegradable material. An example of a suitable gas-generating additive isan aluminum powder that is commercially available from HalliburtonEnergy Services, Inc., of Duncan, Oklahoma, under the tradename “SUPERCBL.” SUPER CBL is available as a dry powder or as a liquid additive.Where included, a gas may be added at the surface to the cementcompositions of the present invention in an amount sufficient to providea gas concentration under downhole conditions in the range of from about0.5% to about 30% by volume of the cement composition. Where included, agas-generating additive may be present in the cement compositions of thepresent invention in an amount in the range of from about 0.1% to about5% bwoc. In certain exemplary embodiments where the gas-generatingadditive is aluminum powder, the aluminum powder may be present in thecement compositions of the present invention in an amount in the rangeof from about 0.1% to about 1% bwoc. In certain exemplary embodimentswhere the gas-generating additive is an azodicarbonamide, theazodicarbonamide may be present in the cement compositions of thepresent invention in an amount in the range of from about 0.5% to about5% bwoc. Where included, the gas or gas-generating additive may be addedto the cement compositions in a variety of ways, including, but notlimited to, dry blending it with the hollow particles, or injecting itinto the cement composition as a liquid suspension while the cementcomposition is being placed within the subterranean formation.

Optionally, the cement compositions of the present invention maycomprise a polymer emulsion comprising at least one polar monomer and atleast one elasticity-enhancing monomer. In certain exemplary embodimentsthe polymer emulsion may further comprise a stiffness-enhancing monomer.As used herein, the term “polymer emulsion” will be understood to mean awater emulsion of a rubber or plastic obtained by polymerization. Such apolymer emulsion is commonly known as “latex,” and the terms “polymeremulsion” and “latex” are interchangeable herein. Generally, the polarmonomer may be selected from the group consisting of: vinylamine, vinylacetate, acrylonitrile, and the acid, ester, amide, and salt forms ofacrylates (e.g., acrylic acid). Generally, the elasticity-enhancingmonomer may be selected from the group consisting of: ethylene,propylene, butadiene, 1,3-hexadiene, and isoprene. In certain exemplaryembodiments that include a stiffness enhancing monomer, the stiffnessenhancing monomer may be selected from the group consisting of: styrene,t-butylstyrene, α-methylstyrene, and sulfonated styrene. Generally, thepolar monomer may be present in the polymer emulsion in an amount in therange of from about 1% to about 90% by weight of the polymer emulsion.Generally, the elasticity-enhancing monomer may be present in thepolymer emulsion in an amount in the range of from about 10% to about99% by weight of the polymer emulsion. When the polymer emulsion furthercomprises a stiffness-enhancing monomer, the stiffness-enhancing monomermay be present in the polymer emulsion in an amount in the range of fromabout 0.01% to about 70% by weight. Varying the amounts of theconstituents of a latex may change the properties of the latex, so as toaffect the type and degree of properties of the cement compositions ofthe present invention that optionally may include such latex. Forexample, when a latex having a high concentration of anelasticity-enhancing monomer (e.g., butadiene), is incorporated into acement composition of the present invention, the elasticity-enhancingmonomer may increase, inter alia, the elastomeric properties of thecement composition. For example, a latex having a high concentration ofa stiffness-enhancing monomer (e.g., styrene), or a polar monomer (e.g.,acrylonitrile), may decrease, inter alia, the elastomeric properties ofthe cement composition. Thus, one of ordinary skill in the art, with thebenefit of this disclosure, will appreciate that the mechanicalproperties of a cement composition may be adjusted by varying theconstituents of a polymer emulsion that may be incorporated in thecement composition. In certain exemplary embodiments, a polymer emulsionmay be added to the cement compositions of the present invention bymixing the polymer emulsion with water, which then may be mixed with ahydraulic cement to form a cement composition. In certain exemplaryembodiments, a polymer emulsion may be added to the cement compositionsof the present invention by evaporating the water from a latex preparedas a water emulsion, thereby forming a dry polymer additive. The drypolymer additive then may be mixed with a hydraulic cement, which thenmay be mixed with water to form a cement composition. An example of asuitable polymer emulsion is an aqueous styrene butadiene latex that iscommercially available from Halliburton Energy Services, Inc., ofDuncan, Oklahoma, under the tradename “LATEX 200OTM.” Where present, thepolymer emulsion may be included within the cement composition in anamount in the range of from about 5% to about 100% by weight of thewater therein. In certain exemplary embodiments, the cement compositionthat comprises a polymer emulsion further may comprise a surfactant,inter alia, to stabilize the polymer emulsion. In certain exemplaryembodiments, the surfactant may be a nonionic ethoxylated nonylphenol.Examples of suitable surfactants are commercially available fromHalliburton Energy Services, Inc., of Duncan, Oklahoma, under thetradenames “STABILIZER 434 B” and “STABILIZER 434 C.” Where included,the surfactant may be present in the cement composition in an amount inthe range of from about 10% to about 20% by weight of the polymeremulsion.

Additional additives optionally may be added to the cement compositionsof the present invention as deemed appropriate by one skilled in the artwith the benefit of this disclosure. Examples of such additives includefluid loss control additives, defoamers, dispersing agents, setaccelerators, salts, formation conditioning agents, weighting agents,set retarders, hollow glass or ceramic beads, elastomers, fibers and thelike. An example of a suitable dispersing agent is commerciallyavailable from Halliburton Energy Services, Inc., of Duncan, Okla. underthe tradename “CFR-3.” An example of a suitable set retarder is alignosulfonate that is commercially available from Halliburton EnergyServices, Inc., of Duncan, Okla. under the tradename “HR®-5.”

In certain exemplary embodiments, the cement compositions of the presentinvention may be prepared by dry blending the degradable materials withthe cement before the addition of water, or by mixing the degradablematerials with water before it is added to the cement, or by mixing thedegradable materials with the cement slurry consecutively with or afterthe addition of water. In certain preferred embodiments, the degradablematerials are dry blended with the cement before the addition of water.In other exemplary embodiments, the degradable materials may bepre-suspended in water and injected into the cement composition, or intothe cement composition as an aqueous slurry, if desired.

An example of a method of the present invention comprises: providing acement composition that comprises a hydraulic cement, and a degradablematerial; placing the cement composition in a subterranean formation,allowing the cement composition to set therein; and allowing thedegradable material to degrade. In certain exemplary embodiments of thepresent invention, the subterranean formation may comprise amultilateral well. In certain exemplary embodiments of the presentinvention, the subterranean formation may comprise a well that comprisesan expandable tubular. Another example of a method of the presentinvention is a method of enhancing the mechanical properties of a cementcomposition comprising adding a degradable material to the cementcomposition, and allowing the degradable material to degrade.

To facilitate a better understanding of the present invention, thefollowing examples of some of the preferred embodiments are given. In noway should such examples be read to limit, or to define, the scope ofthe invention.

EXAMPLE 1

Sample cement compositions were prepared in accordance with APIRecommended Practice 10B, Twenty-Second Edition, 1997.

Sample Composition No. 1 comprised Class A cement and about 37.85% waterbwoc. Sample Composition No. 1 was cured for 1 day at a temperature of210° F. and a pressure of 1000 psi.

Sample Composition No. 2 comprised Class A cement, about 8% bwocpolylactic acid (“PLA”) in bead form (about 0.75 mm in diameter), andabout 37.85% water bwoc. Sample Composition No. 2 was cured for 1 day ata temperature of 210° F. and a pressure of 1000 psi. The density ofSample Composition No. 2, upon setting, was measured at 16.4 lb/gallon.

Sample Composition No. 3 comprised Class A cement, PLA at about 8% bwocand about 37.85% water bwoc. Sample Composition No. 3 was cured for 14days at a temperature of 210° F. and a pressure of 1000 psi. The densityof Sample Composition No. 3, upon setting, was measured at 16.3lb/gallon.

The sample cement compositions were cured in 2″×2″×2″ brass moldsaccording to the API procedure for measuring compressive strengths usingTinius Olsen (TO) Instrument. The crushed samples were examined for thepresence of bubble structure. The results of the testing are set forthin the table below. TABLE 1 Degradable Observed cement materialCompressive matrix Composition (% bwoc) strength (psi) morphology SampleNone 5290 solid Composition No. 1 Sample PLA 2220 Hollow bubblesComposition 8% bwoc No. 2 Sample PLA 4780 Hollow bubbles Composition 8%bwoc No. 3

The above example demonstrates, inter alia, that the inclusion in acement composition of degradable material in bead form contributes tothe formation of voids upon degradation and sorption of the degradablematerial.

EXAMPLE 2

The Young's Modulus of Sample Composition Nos. 1 and 2 was determinedafter each Sample Composition had cured. The Young's Modulus wasmeasured by performing Load vs. Displacement measurements on MTS LoadFrame equipment under unconfining conditions. The results are set forthin Table 2. TABLE 2 Degradable material Composition (% bwoc) Young'sModulus (psi) Sample Composition None   2 × 10⁶ No. 1 Sample CompositionPLA 1.22 × 10⁶ No. 2 8% bwoc

The above example demonstrates, inter alia, that the cement compositionsof the present invention possess improved elasticity and resiliency.

EXAMPLE 3

Sample cement compositions were prepared that comprised Class A cement,about 0.375% bwoc CFR-3 dispersant, PLA at about 15% bwoc, and about33.8% water bwoc. The sample cement compositions were cured for 3 daysat a temperature of 190° F. and a pressure of 1000 psi.

Sample Composition No. 4 did not comprise a retarder.

Sample Composition No. 5 comprised about 0.5% bwoc HR®-5 retarder.

Sample Composition No. 6 comprised about 1.0% bwoc of HR®-5 retarder.

Thickening times at 190° F. were measured according to API procedure.The results of the testing are set forth in the table below. TABLE 3Retarder concentration Thickening Time Sample Composition (% bwoc)(hrs:min) Sample Composition No. 4 0 0:31 Sample Composition No. 5 0.51:00 Sample Composition No. 6 1.0 1:18

The above example suggests, inter alia, that the compositions of thepresent invention can be designed to set at a desired time.

EXAMPLE 4

Two sample compositions were prepared that comprised Class A cement,about 0.375% bwoc CFR-3 dispersing agent, about 15% PLA bwoc and about33.8% water bwoc. Both sample compositions were cured at a temperatureof 190° F. and a pressure of 1000 psi.

Sample Composition No. 7 cured for three days.

Sample Composition No. 8 cured for five days.

The compressive strength of each sample composition was measuredaccording to the API procedure for measuring compressive strengths usingTinius Olsen (TO) Instrument. The results are set forth in the tablebelow. TABLE 4 Compressive Composition Cure time (days) strength (psi)Sample 3 2890 Composition No. 7 Sample 5 3080 Composition No. 8

The above example demonstrates, inter alia, that the cement compositionsof the present invention have suitable compressive strengths for, interalia, oil well cementing.

EXAMPLE 5

Three sample cement compositions were prepared comprising Class A cementand about 37.85% water bwoc. The sample cement compositions were curedat a temperature of 210° F. and a pressure of 1000 psi.

Sample Composition No. 9 was formulated to have a design slurry densityof 16.5 ppg. Sample Composition No. 9 comprised no degradable material,and was cured for 1 day.

Sample Composition No. 10 was formulated to have a design slurry densityof 15.85 ppg. Sample Composition No. 10 comprised about 8% bwocpolyvinyl acetate in the bead form, and was cured for 1 day.

Sample Composition No. 11 was formulated to have a design slurry densityof 15,85 ppg. Sample Composition No. 11 comprised about 8% bwocpolyvinyl acetate in the bead form, and was cured for 6 days. TABLE 5Design Degradable Slurry Cure Compressive Cement Material Density SetDensity Time strength Matrix Composition (% bwoc) (ppg) (ppg) (days)(psi) Morphology Sample None 16.5 Not 1 day 6230 Solid CompositionDetermined No. 9 Sample Polyvinyl 15.85 16.25 1 day 3960 Solid polymerComposition acetate, 8% beads No. 10 bwoc Sample Polyvinyl 15.85 16.21 6 days 3770 Hollow bubbles Composition acetate, 8% and polymer No. 11bwoc beads

The above example demonstrates, inter alia, that the cement compositionsof the present invention may be designed to have desired rates ofdegradation.

EXAMPLE 6

A 16.5 ppg slurry was prepared using Class H cement and 37.85% bwocwater. The slurry was divided into two portions of 300 ml each. Aconventional lost-circulation material (FLOCELE flakes, available fromHalliburton Energy Services, Inc., of Duncan, Oklahoma) was added to oneportion in the amount of 0.2% bwoc. To the other portion, flakes ofpolylactic acid film having a thickness of 15-30 microns were added.Both portions were stirred for four hours at room temperature, andvisually inspected for disappearance of the additive. In both cases, theflakes persisted without undergoing degradation. The above exampledemonstrates, inter alia, that degradable materials may be added tocement compositions in order to prevent loss of the cement compositionto permeable zones in the formation, such as zones comprising fractures.

Therefore, the present invention is well adapted to carry out theobjects and attain the ends and advantages mentioned as well as thosethat are inherent therein. While the invention has been described and isdefined by reference to exemplary embodiments of the invention, such areference does not imply a limitation on the invention, and no suchlimitation is to be inferred. The invention is capable of considerablemodification, alteration, and equivalents in form and function, as willoccur to those ordinarily skilled in the pertinent arts and having thebenefit of this disclosure. The depicted and described embodiments ofthe invention are exemplary only, and are not exhaustive of the scope ofthe invention. Consequently, the invention is intended to be limitedonly by the spirit and scope of the appended claims, giving fullcognizance to equivalence in all respects.

1. A cement composition comprising a hydraulic cement, water sufficientto form a pumpable slurry, and a degradable material, wherein thedegradable material is selected from the group consisting of aliphaticpolyesters, poly(lactides), poly(glycolides), poly(ε-caprolactones),poly(hydroxybutyrates), poly(anhydrides), aliphatic polycarbonate, orthoesters, poly(orthoesters), poly(vinylacetates), polyamides, proteins,polyaminoacids, nylons, poly(caprolactams), polylactic acid, celluloseacetate, and combinations thereof.
 2. The cement composition of claim 1wherein the hydraulic cement is selected from the group consisting ofPortland cements, pozzolanic cements, gypsum cements, high aluminacontent cements, phosphate cements, silica cements, and high alkalinitycements.
 3. The cement composition of claim 1 wherein the degradablematerial comprises a material that degrades at a desired time aftercontact with the cement composition.
 4. The cement composition of claim1 wherein the degradable material comprises a material that preventsfluid loss into the subterranean formation.
 5. The cement composition ofclaim 1 wherein the degradable material, upon degradation, forms atleast one gas, salt or combination thereof.
 6. The cement composition ofclaim 1 wherein the cement composition further comprises an additiveselected from the group consisting of fluid loss control additives,defoamers, dispersing agents, set accelerators, salts, formationconditioning agents, weighting agents, set retarders, ceramic beads,glass beads, elastomers, and combinations thereof.
 7. The cementcomposition of claim 1 wherein the degradable material comprisesparticles in the form of a thin film, a flake, a substantially sphericalparticle, a bead, a fiber, or a combination thereof.
 8. The cementcomposition of claim 1 wherein the degradable material is present in thecement composition in an amount sufficient to leave voids in the setcement that enhance the mechanical properties of the set cement.
 9. Thecement composition of claim 1 wherein the degradable material is presentin the cement composition in an amount in the range of from about 1% toabout 25% by weight of cement.
 10. The cement composition of claim 1wherein the cement composition further comprises a polymer emulsion,wherein the polymer emulsion is present in the cement composition in anamount in the range of from about 5% to about 100% by weight of anamount of water in the cement composition.
 11. The cement composition ofclaim 10 wherein the polymer emulsion comprises a polar monomer selectedfrom the group consisting of vinylamine, vinyl acetate, acrylonitrile,and the acid, ester, amide, and salt forms of acrylates, and at leastone elasticity-enhancing monomer selected from the group consisting ofethylene, propylene, butadiene, 1,3-hexadiene, and isoprene.
 12. Thecement composition of claim 11 wherein the polar monomer is present inthe polymer emulsion in an amount in the range of from about 1% to about90% by weight of the polymer emulsion.
 13. The cement composition ofclaim 11 wherein the at least one elasticity-enhancing monomer ispresent in the polymer emulsion in an amount in the range of from about10% to about 99% by weight of the polymer emulsion.
 14. The cementcomposition of claim 11 wherein the polymer emulsion further comprises astiffness-enhancing monomer.
 15. The cement composition of claim 14wherein the stiffness-enhancing monomer is selected from the groupconsisting of styrene, t-butylstyrene, α-methylstyrene, and sulfonatedstyrene, and is present in the polymer emulsion in an amount in therange of from about 0.01% to about 70% by weight of the polymeremulsion.
 16. The cement composition of claim 10 wherein the polymeremulsion is an aqueous styrene butadiene latex.
 17. The cementcomposition of claim 10 wherein the cement composition further comprisesa surfactant, wherein the surfactant is present in the cementcomposition in an amount in the range of from about 10% to about 20% byweight of the polymer emulsion.
 18. The cement composition of claim 1wherein the cement composition comprises a gas, wherein the gas ispresent in the cement composition in an amount sufficient to provide agas concentration in the range of from about 0.5% to about 30% by volumeof the cement composition, measured when the cement composition has beenplaced in a subterranean formation.
 19. The cement composition of claim1 wherein the cement composition comprises a gas-generating additiveselected from the group consisting of an aluminum powder andazodicarbonamide, wherein the gas generating additive is present in thecement composition in an amount in the range of about 0.1% to about 5%by weight of the cement.
 20. The cement composition of claim 1 whereinthe cement is a Portland cement; wherein the degradable material ispolylactic acid, wherein the polylactic acid is present in the cementcomposition in an amount in the range of about 1% to about 25% by weightof the cement; and wherein the water is present in the cementcomposition in an amount in the range of from about 25% to about 150% byweight of the cement.